fix(linux/build): handle DooD bind-mount in CI (M1.1)

build.sh ran fine locally but failed in Gitea Actions on the first
reproducibility-gated run (#4250) with:

    bash: line 3: /work/linux/build/config/silvermetal-base.conf:
    No such file or directory

Root cause: classic Docker-out-of-Docker confusion. build.sh runs
inside the act_runner job container, which talks to the host's docker
daemon via the mounted /var/run/docker.sock. The "-v ${REPO_ROOT}:/work"
flag was being interpreted by the host daemon against the host
filesystem, where /workspace/SilverLABS/SilverMetal does not exist;
docker silently auto-created an empty dir there and mounted that as
/work, so the config source target was missing.

Fix: detect GITHUB_ACTIONS and use --volumes-from "$(hostname)" in CI
to inherit the parent job container's /workspace mount intact. Locally
we keep a bind mount, but use the same path inside and outside
(${REPO_ROOT}:${REPO_ROOT}) so the inner heredoc is identical in both
modes. Inner script now references "${REPO_ROOT}/..." and
"${BUILD_DIR}/..." instead of the synthetic /work and /out paths.

No reproducibility implications — bind topology doesn't affect bytes
inside the ISO.

Verified locally: bash -n passes; structural change only, behaviour
preserved for the non-CI path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

linux/build/scripts/build.sh

@@ -76,26 +76,55 @@ mkdir -p "${BUILD_DIR}"
 echo "build.sh: commit=${COMMIT_SHA} epoch=${SOURCE_DATE_EPOCH} snapshot=${SNAPSHOT_TIMESTAMP}"
 echo "build.sh: output -> ${BUILD_DIR}"
 
+# --- Mount strategy: local vs CI -------------------------------------------
+# Locally we bind-mount the repo into the build container at the *same*
+# path (self-referential), so internal references work transparently and
+# the inner script doesn't need to care which host it's on.
+#
+# In CI we can't do that. build.sh runs inside a Gitea Actions job
+# container which talks to the *host's* docker daemon via /var/run/docker.sock.
+# Bind-mounting REPO_ROOT (= /workspace/<owner>/<repo>) would resolve
+# against the host filesystem where that path doesn't exist; docker
+# silently creates an empty dir on the host and mounts that, leaving the
+# build container with an empty /work and a confusing "No such file or
+# directory" error on the first config source.
+#
+# The standard fix for that DooD topology is --volumes-from of the parent
+# job container, which inherits its /workspace mount intact. That keeps
+# paths identical inside and outside, so the inner heredoc below is the
+# same in both environments.
+if [[ -n "${GITHUB_ACTIONS:-}" ]]; then
+    BIND_ARGS=(--volumes-from "$(hostname)")
+else
+    BIND_ARGS=(-v "${REPO_ROOT}:${REPO_ROOT}:rw")
+    # If BUILD_DIR lives outside REPO_ROOT (uncommon, but the env-var
+    # override allows it), mount it explicitly too.
+    if [[ "${BUILD_DIR}" != "${REPO_ROOT}/"* && "${BUILD_DIR}" != "${REPO_ROOT}" ]]; then
+        BIND_ARGS+=(-v "${BUILD_DIR}:${BUILD_DIR}:rw")
+    fi
+fi
+
 # --- Run the build inside the container ------------------------------------
 # --privileged is required because live-build mounts loop devices and chroots.
 # --network=host lets the container reach snapshot.debian.org without us
 # fighting CI proxy config; tighten if/when that becomes a concern.
 docker run --rm --privileged \
     --network=host \
+    "${BIND_ARGS[@]}" \
     -e SOURCE_DATE_EPOCH \
     -e SNAPSHOT_TIMESTAMP \
     -e LC_ALL=C.UTF-8 \
     -e LANG=C.UTF-8 \
     -e TZ=UTC \
-    -v "${REPO_ROOT}:/work:rw" \
-    -v "${BUILD_DIR}:/out:rw" \
-    -w /work \
+    -e REPO_ROOT="${REPO_ROOT}" \
+    -e BUILD_DIR="${BUILD_DIR}" \
+    -w "${REPO_ROOT}" \
     "${BUILDER_IMAGE}" \
     bash -euo pipefail -c '
         # shellcheck disable=SC1091
-        source /work/linux/build/config/silvermetal-base.conf
+        source "${REPO_ROOT}/linux/build/config/silvermetal-base.conf"
 
-        cd /work/linux/build/derivative-maker
+        cd "${REPO_ROOT}/linux/build/derivative-maker"
 
         ./derivative-maker \
             --build \
@@ -103,18 +132,19 @@ docker run --rm --privileged \
             --flavour "${DERIVATIVE_FLAVOUR}" \
             --arch "${DERIVATIVE_TARGET_ARCH}" \
             --dist "${DERIVATIVE_DIST}" \
-            --config /work/linux/build/config/silvermetal-base.conf
+            --config "${REPO_ROOT}/linux/build/config/silvermetal-base.conf"
 
-        # derivative-maker writes into its own build/ dir; collect into /out.
-        # Exact upstream output paths can shift between tags — keep this
-        # tolerant. Anything matching *.iso under the tree is what we want.
+        # derivative-maker writes into its own build/ dir; collect into
+        # BUILD_DIR. Exact upstream output paths can shift between tags —
+        # keep this tolerant. Anything matching *.iso under the tree is
+        # what we want.
         find . -maxdepth 6 -type f -name "*.iso" -print0 \
-            | xargs -0 -I{} cp -av "{}" /out/
+            | xargs -0 -I{} cp -av "{}" "${BUILD_DIR}/"
 
         # Manifest of file metadata that lives inside the ISO. Useful when
         # diagnosing reproducibility regressions without re-extracting.
         find . -maxdepth 6 -type f -name "*.manifest" -print0 \
-            | xargs -0 -I{} cp -av "{}" /out/ 2>/dev/null || true
+            | xargs -0 -I{} cp -av "{}" "${BUILD_DIR}/" 2>/dev/null || true
     ' || { echo "build.sh: derivative-maker failed"; exit 3; }
 
 # --- Hash artefacts ---------------------------------------------------------